A cavity resonance phenomenon that occurs in an airtightly sealed tire cavity formed between the rim of a wheel and a pneumatic tire mounted on the rim is a major factor of tire noise. Noise around 250 Hz regularly heard during running or impulsive sounds generated in running over joint lines or the like of a road, for example, is involved with the cavity resonance phenomenon.
As a technique for reducing noise caused by such a cavity resonance phenomenon, there is known, for example, a technique such that sound absorbing members of a porous material such as urethane resin foam are disposed at two opposed locations in the circumferential direction of a tire on the inner surface of the tread portion facing the tire cavity so as to vary the cross-sectional area of the tire cavity in the circumferential direction of the tire (see a patent document 1, for example). Such a variation of the cross-sectional area of the tire cavity in the tire circumferential direction makes resonant frequency varied with rotation of the tire every second, and a sound absorbing effect is obtained by the sound absorbing members, whereby cavity resonance noise is effectively reduced. Further, by increasing the density of the sound absorbing members, the mechanical strength of the sound absorbing members is enhanced, obtaining durability.
However, a pneumatic tire having such a tire noise reduction device is uneven in mass distribution in the circumferential direction of the tire. Therefore, a problem is that tire vibration is encouraged during running of a vehicle. In addition, with noise reduction of a vehicle, it is strongly required to further reduce tire noise.
Patent Document 1: Japanese Patent Application Kokai Publication 2003-226104